HTTP

Note See MDN

CORS

CORS

In php to allow Multiple Sites Access. (As always header before any other output from php !)

$http_domain_origin = $_SERVER[ 'HTTP_ORIGIN' ];

if ( $http_domain_origin == "http://www.site1.com" || $http_origin == "http://www.site2.com" || $http_origin == "http://www.site3.com" ) // # etc. #
{  
    header( "Access-Control-Allow-Origin: $http_domain_origin" );
}